1. Field of the Invention
This invention relates to a telephone system for emergency calls and more particularly to a system which facilitates the administration of the call capacity of a public safety answering point (PSAP) and provides last chance routing of emergency calls after an attempt to route such calls to designated and alternate PSAPs.
2. Discussion of Prior Art
In the field of telephony, equipment and services for routing emergency telephone calls (911 Calls) have been associated with the universal telephone number (TN) "9-1-1", abbreviated herein as "911" and referred to as an emergency service number ("ESN"). These equipment and services are herein respectively designated "911 Equipment" and "911 Services".
Prior 911 Equipment has generally been provided for large metropolitan areas which are served by a public switched telephone network (PSTN) generally having more than one-hundred fifty thousand subscriber lines. The PSTN is divided into service areas, each of which may have over 150,000 subscriber lines. Each subscriber's telephone number (TN) in a particular service area is assigned to a particular ESN, and is served by a specific end office (EO). The EO routes a 911 Call that is on a particular one of its subscriber lines to the 911 Equipment via trunks. The trunks are generally capable of carrying automatic number identification ("ANI") codes. Those trunks having such capability are referred to herein as "ANI trunks." ANI code is in the form of eight bits, including a seven digit TN and one information bit that represents the numbering plan digit, or area code, within which the call originated.
Each ESN represents a geographic region within the service area where all subscribers in that region are served by the same primary group of emergency service agencies (ESPs). The groups could, for example, include a fire department, a police department and a hazardous material recovery department of a particular municipality.
In the past, the 911 Equipment has been used to provide 911 Services. The 911 Services are separately provided for each service area by PSAP equipment which responds to 911 Calls having the same ESN. Because so many subscriber lines (e.g., over 150,000) are served by the EO for a given urban service area, the PSAP for the given urban service area (an "urban PSAP") is staffed by attendants on a twenty-four hour a day basis. Such an urban PSAP is generally always ready to receive 911 Calls, and is thus generally always "active", as compared to a PSAP which has suspended its 911 Services and which is referred to as being "inactive."
Attendants are people who are trained to handle 911 Calls using the particular 911 Equipment at the PSAP according to the procedures that have been established at that PSAP. Such procedures may include how the PSAP is designated. A PSAP may be designated "first choice" or "alternate", which refers to the order in which 911 Calls are directed to the PSAP to be answered. First choice PSAPs are the first PSAPs which should receive 911 Calls from the service area. Alternate PSAPs are PSAPs which receive 911 Calls when some event prevents the first choice PSAP from receiving the 911 Call. For example, the alternate PSAP may receive the 911 Call as a transfer from a first choice PSAP or directly from subscribers or the public via subscriber lines when the first choice PSAP to which the 911 Call is assigned is experiencing power failure, doesn't answer the 911 Call within a specified time, or when all routes to the first choice PSAP are busy. These and other situations at the first choice PSAP result in overflow of 911 Calls to the alternate PSAP.
The PSTN includes a feature that automatically provides the caller's ANI Code. When a 911 Call is received at a PSAP via an ANI Trunk, it will be received with the ANI code.
An ALI/DMS also includes a feature that automatically provides the caller's address and other pertinent details, referred to as the automatic location identification (ALI) feature. Via ALI, when a 911 Call is received at a PSAP it is received with such details, which is referred to as the ALI code. However, in existing 911 Equipment the database for producing the ALI code is generally stored in the PSTN at a central location for a very wide geographic area. For example, for the states of Arizona, Colorado, New Mexico and Wyoming in one PSTN, the ALI database for the ALI code for a relatively small area in Wyoming is stored in Denver, Colorado. To provide the ALI Code for a 911 Call in that area in Wyoming, a long distance call must be made to Denver, and redundant long distance lines must be kept available to assure reliability. This increases the cost and time required to provide the ALI code for 911 Calls in remote areas.
911 Equipment known to Applicants provides incoming 911 Call data to the PSAPs in the form of a string of data. This data is unrelated to the first choice PSAP and to the trunk that carries the 911 Call to the 911 Equipment. The ALI code is obtained from an ALI/DMS host which is part of the PSTN. Neither the ALI/DMS nor the 911 Equipment organizes that code into a format that is efficiently and quickly useful to the PSAP attendant for determining why the 911 Call was not properly and quickly routed to the first choice PSAP or another PSAP.
In one example of prior 911 Equipment, only information identifying the trunk that is carrying the 911 Call is sent to the first choice PSAP. A remote system provides for the display of the ANI and NPD of the incoming 911 Call. However, because such system is remote from the first choice PSAP, problems arise from queuing and transmission delays.
911 Equipment known to Applicants includes that used in connection with the trademark "1A ESS" by The American Telephone & Telegraph Company ("1A 911 Equipment"), which is used to provide "E9-1-1" service. In the IA 911 Equipment, in an endeavor to answer all incoming 911 Calls, when a particular 911 Call has not been initially routed to a PSAP or other transfer point the 1A 911 Equipment searches a "link" list of destinations to which a particular 911 Call could possibly be routed. These destinations are searched on a most logical basis, and only include destinations (such as alternate PSAPs) that were previously "searched" in a prior attempt to route the 911 Call. The search loops once through the link list.
Problems with such logical searches of previously searched destinations are that:
(1) the link list, being based on previously searched destinations, may omit destinations which are available to answer the 911 Call, but which are not searched because they are not on the link list, and PA0 (2) if the search of the link list fails to locate an available destination, an attempt is made to route the 911 Call to a default destination, and if that is not available, to disconnect the call. As a result, only one additional search of the link list is conducted prior to disconnecting the 911 Call, which tends to increase the probability of disconnecting the 911 Call.
Additionally, many COs provide incoming calls to the 1A 911 Equipment and incoming trunks are connected from the 1A 911 Equipment to an ANI facility. Significantly, the number of such incoming trunks limits the number of calls that may be sent to the ANI facility. Because the ANI facility is then connected to the key of the 1A 911 Equipment the number of calls available to such key is limited.